1. Field of the Invention:
This invention relates generally to machine tool assemblies and components therefor, such as hollow rotary spindle shafts and draw bar assemblies for use with machine tools in the performance of various types of operations such as milling, drilling, boring, tapping, and the like. The invention is particularly concerned with a machine tool high speed hollow rotary spindle shaft in which is mounted an axially movable, nonrotational draw bar assembly that includes a tool holder gripping means. The draw bar assembly, with the tool gripping means, functions to draw a tool holder into a tight frictional engagement with a seat formed in a tapered bore in the spindle shaft, and the draw bar assembly with the tool holder gripping means rotates with the spindle shaft.
2. Description of the Prior Art
It is known in the machine tool art to provide hollow rotary spindle shafts in which are releasably mounted tool holders, or the like, that are held in position by a draw bar assembly that includes a tool holder gripping means, such as a collet means. The draw bar assembly is generally biased by springs to an operative position so as to secure a tool holder in a seat in a tapered bore in the spindle shaft by frictional engagement. The prior art draw bar assemblies are actuated from an operative tool holder retaining position to an inoperative tool holder releasing position against the bias of the springs by a suitable actuator means, such as a hydraulic motor, or the like, when it is described to remove a tool holder from a spindle shaft. Examples of the aforedescribed machine tool spindle shafts and draw bar assemblies and tool holder gripping means are illustrated in U.S. Pats. Nos. 3,516,149, 3,520,228, and 3,875,848.
A problem and disadvantage of the prior art draw bar assemblies and tool holder gripping means is that they exert a thrust on a rotary spindle shaft when the spindle actuator means is operated. The last mentioned thrust force on a spindle shaft is injurious to the spindle shaft high speed bearings. A further problem with the prior art draw bar assemblies and tool holder gripping means is that there is no way of indicating to the operator, when the draw bar assembly is moved to the operative position, and as to whether or not a new tool holder has been moved into position in the spindle shaft or whether or not the tool holder gripping means is broken.